Introduction to the Cephalorhyncha

Priapulids, Kinorhynchs, and Loriciferans

The Cephalorhyncha are a group of ecdysozoan
animals that traditionally have been
classified together with a variety of unusual and lesser known organisms
in a larger group variously known as the Aschelminthes, Nemathelminthes, and/or
Pseudocoelomata. This larger group is now known to be polyphyletic, and
contained several taxa that were not closely related. However, one part
of that group -- the priapulids, kinorhynchs, and loriciferans -- appears
to form a group in its own right. Now these organisms are grouped with the
nematodes and arthropods based on a set of shared characters including
the presence of a cuticle and the fact that they periodically shed
their cuticle in a process called ecdysis. Some zoologists have
suggested an even closer relationship with the nematodes, and call the
combination of nematodes and cephalorhynchs the Cycloneuralia.

All cephalorhynchs have a spiny proboscis which can be collapsed
inside the head or everted (turned inside out) to gather food using the
spines. This is rather like turning a sock inside out, only with the
surface of the sock covered with numerous tiny hooks. It is this spiny
evertible proboscis that gives the group its name Cephalorhyncha, meaning
"beak"-head.

Though cephalorhynchs may abound in marine sands, muds, or gravel, none
of the members of the Cephalorhyncha are particularly diverse today, and
most are tiny (less than 1 mm long) so they are unlikely to be seen or
encountered. Additionally, neither the Kinorhycha nor Loricifera have
any fossil record. So you might be asking yourself, "Why is there a whole page
about this group on a site devoted to paleontology?" The answer lies in
the fossil record of the third group, the Priapulida. Though there are only
about 16 species of priapulid worm today, quite a number of fossils of the
carnivorous priapulid Ottoia have been found in the
Burgess Shale.
In fact, priapulids and arthropods are the most common
complex animal fossils from this Cambrian
locality, and make their appearance in the fossil record as early as most
of the first-known members of various major animal groups.

Modern priapulids burrow in marine sediments and many be several centimeters
long, though most are microscopic. Larger priapulids primarily inhabit
cold waters at hish latitudes, where they hunt for polychaete worms.
There are 16 species known to be alive at present.

The Kinorhyncha are microscopic, spiny-headed worms. You can see a greatly
enlarged diagram of one at right on this page. Kinorhynchs feed on
bacteria and tiny particles associated with sand grains. They are the most
diverse group of cephalorhynchs today, with about 150 species.
Loricifera is a recently discovered group of animals. Discovered in 1983,
these tiny worms have a spiny head, and so resemble the kinorhynchs to
which they are related. Only about 10 species have so far been described.